Aqueous dispersion of resin is widely used in coatings, adhesives, fiber processing, sheet or film processing and paper processing and is superior in handling and working environment safety, compared to a solvent solution. Almost all resins in the existing aqueous dispersion are non-biodegradable, with the exception of starch and rubber latex, and there is a problem that the load to the environment is large, in terms of waste treatment.
In recent years, environmental problems caused by waste plastics have been drawing interest and while realization of a recycling society on a global scale is desired, a biodegradable plastic, which is decomposed into water and carbon dioxide after the plastic is used by function of microorganisms, is attracting attention. Sheets, films, fibers and molded articles have already been manufactured both domestically and internationally, but aqueous dispersion of resin referred to as emulsion or latex which are biodegradable are few in number.
For example, an aqueous composition for paper coating containing polylactic acid is prepared by solidifying resin after melting by heat in a solvent and further, stirring together with glass beads at a high speed and then pulverizing, in order to obtain an aqueous dispersion of fine particles (see JP-A-9-78494, paragraph 0008). Also, an aqueous dispersion of aliphatic polyester emulsion is prepared by mixing and kneading melted resin and an aqueous solution of an emulsifier (see. JP-A-11-92712, paragraph 0006, JP-A-2001-354841, paragraph 0006). Also, there is a method of pulverizing frozen resin, but all of the methods require a great deal of labor in order to obtain an aqueous dispersion of fine particles and are economically disadvantageous.
As examples of polyhydroxy alkanoate (hereinafter referred to as PHA) produced from a microorganism, a homopolymer of 3HB (hereinafter referred to as PHB) and a copolymer of 3HB and 3-hydroxy valylate (hereinafter referred to as 3HV) (hereinafter referred to as PHBV) are disclosed. However, these resins have the faults of little elongation and being brittle (see. U.S. Pat. No. 5,451,456, column 4, lines 5 to 23, and JP-A-11-500613, page 7, lines 21 to 22). Consequently, a coating film formed by applying an aqueous dispersion containing such resins also has little elongation and is brittle and when applied to sheet, film or paper, there is the problem of cracks, which occur by folding.
The object of the present invention is to provide an aqueous dispersion of biodegradable polyester, which has excellent film forming properties and provides a resin coating film which is flexible, has favorable elongation and is strong to folding, when applied in coatings, adhesives, fiber processing, sheet film processing and paper processing. Also, the present invention aims to provide a process for easily preparing an aqueous dispersion of fine particles of biodegradable polyester, without solidifying resin after melting by heat in a solvent, without mixing and kneading melted resin and an aqueous solution of an emulsifier or without pulverizing frozen resin.